Just as with anti-reflection (AR) coatings, metallic mirror coatings are also designed for different regions of the spectrum. Edmund Optics® offers a series of protected metallic coatings capable of providing high reflectance values for applications using wavelengths ranging from 250nm to beyond 10μm. Our standard metallic mirror coatings include Protected Aluminum, Enhanced Aluminum, UV Enhanced Aluminum, Protected Gold and Protected Silver. Protected Aluminum and Enhanced Aluminum are typically used for visible applications. UV Enhanced Aluminum can be used for UV and visible applications. Protected Gold offers high reflectance for Infrared or near-Infrared wavelengths. Protected Silver provides the highest reflectance between 500-800nm but is best suited as a rear surface reflector due to its sensitivity to tarnishing.
INTRODUCTION TO FIRST SURFACE AND SECOND SURFACE MIRRORS
Edmund Optics® offers a variety of mirror substrates and geometries coated with our standard metallic mirror coatings, laser-line coatings, as well as, custom broadband, narrowband, single laser line, dual laser line and laser-line beamsplitter coatings. All of our mirror products are first surface mirrors, Figure 1, featuring a high reflectivity coating, such as metallic or multi-layer dielectric, deposited on one surface of the glass substrate (we also offer a few families of metal substrate mirrors). By contrast, second surface mirrors, Figure 2, are manufactured in the same manner as first surface mirrors but feature an additional cover glass on the coated surface, usually silver-plated, for protection. This cover glass protects the coating layer from scratching and oxidization. Edmund Optics recommends first surface mirrors to second surface mirrors for the following three reasons (Figure 2):
- Light incident on a second surface mirror is subject to dispersion from reflection off the cover glass. Glass tends to disperse light, causing different wavelengths to refract at different angles.
- Due to Fresnel reflections at the cover glass, reflected light exhibits ghost images, indicated by the dashed red line.
- Due to Fresnel reflections, responsible for approximately 4% reflection loss at each interface, reflection efficiency is reduced by using a second surface mirror.